One of the primary uses for fractionation apparatus is to remove moisture from water vapor, although the fractionation process can be applied to separating many oher fluid components from fluid or gas mixtures. Fractionating apparatus utilize an adsorbent or dessicant bed which adsorbs the desired component from the flow of the fluid mixture passed through the dessicant bed, leaving only a processed fluid free of the adsorbed component to exit the fractionation apparatus. Once the adsorbing capacity of the dessicant bed has been reached, it is necessary to regenerate the dessicant bed by purging it of the adsorbed component. Regenerating the dessicant bed by purging it of the adsorbed component readies or renews the dessicant for renewed adsorption.
A "twin tower" fractionating apparatus is quite commonly employed to obtain the capacity for continuous adsorption of the desired component from a continual stream or flow of the inlet gas or fluid mixture. A twin tower fractionating apparatus uses two separate dessicant beds, each of which is arranged in its own fractionating tower. The inlet mixture is directed through a first one of the dessicant beds to adsorb a predetermined component in an adsorbing phase, while the other or second dessicant bed is regenerated by purging it of the adsorbed component in a desorbing phase. Once the second bed is regenerated and before the first bed has become totally saturated with the adsorbed component, the inlet mixture is switched to flow through the second bed in the other tower. While the mixture is flowing through the second bed in an adsorbing phase, the first dessicant bed is regenerated during a desorbing phase by purging it of the previously adsorbed component. The inlet mixture is switched back and forth between the two dessicant beds and the two separate fractionating towers in this manner to create an alternating cyclic operation. One bed is in an adsorbing phase while the other bed is in a desorbing phase, and the phase conditions of the two dessicant beds in the twin tower apparatus are respectively opposite of one another. This cyclic operation provides a continuous flow of the processed exit fluid which is free of the adsorbed component.
Regenerating the adsorbent bed can be achieved by changing the pressure ambient to the bed, changing the temperature of the bed, changing the concentration of the adsorbed component in the gas phase surrounding the bed, or by combining these approaches. One of the most common regenerating techniques is to bleed a small portion of the processed exit gas flow from the adsorbing tower at a low pressure and flow rate back and in the reverse or counterflow direction through the other dessicant bed. Since the processed exit gas is free of the adsorbed component, the low pressure, low rate of counter flowing gas becomes a purge gas which takes up the adsorbed component and thus regenerates the bed. The purge gas which has taken up the adsorbed component is thereafter discharged from the fractionation apparatus as waste.
The most common approach to controlling the alternation between the adsorption and regeneration phases is a simple timing cycle technique. The maximum amount of time for adsorption is selected, and the phase alternation or switching occurs at each of these time intervals. Controlling the switching based on a predetermined phase timing schedule usually offers reliability of operation due to the relatively simplistic nature of the control system. The simplicity of the pure timing control system creates certain drawbacks, however. For example, should the flow rate of the inlet mixture increase, or should the percentage of the component to be adsorbed in the mixture increase, the dessicant bed may become saturated before the phase timing interval lapses and switching occurs. Once the dessicant bed is fully saturated, it is ineffective to adsorb any more of the desired component. In this condition, known as "crashing," the exit fluid flow from the saturated bed carries the component desired to be adsorbed. This, of course, is contrary to the purpose sought to be achieved by fractionation and results in a breakdown of the desired operation.
To avoid the likelihood of saturating the dessicant bed prior to switching, the phase cycling time is usually adjusted to be conservatively short, thereby achieving some degree of assurance against crashing. The disadvantage of short phase cycle times is that it increases the frequency and thus the number of adsorption-desorption cycles which the dessicant bed must undergo. The lifetime of the dessicant is inversely related to the number of adsorption-desorption cycles it must undergo. Once the dessicant has become relatively non-adsorbent after repeated adsorption-desorption cycles, it is necessary to replace the dessicant in the fractionating tower. Failing to maximumly utilize the full capacity of the dessicant prior to cycling it into regeneration thus results in inefficient utilization and increased costs per given volume of exit fluid which has been processed.
Another drawback to the fixed cycle time operation is that it is usually wasteful. Since a percentage (usually about 13%) of the volume of the processed exit gas flow is used to create the purge gas flow during regeneration, the net yield of processed gas is less than the maximum amount of gas processed by an amount equal to the purge gas volume. With fixed phase time cycling, the purge gas flow usually occurs continually throughout the whole adsorption phase, even after the dessicant bed has been fully desorbed. Energy is thus unnecessarily consumed by processing the excessive purge gas which is lost as waste.
Variable cycle time control systems have also been devised to avoid many of the deficiencies common to fixed cycle time control systems. Complex variable cycle time controllers typically employ sophisticated and technically intricate mioroprocessors and multiple variable sensors. Such systems commonly obtain data relating to the inlet gas flow rate, pressure and temperature; the purge gas inlet and exhaust pressures and temperatures; the exit gas pressure and temperature; and the dew point of the exit gas. By sensing these variables, operating limits are established to optimally control the fractionation. Of course, the disadvantages to such elaborate control systems are higher costs and greater difficulties in servicing and maintaining such systems.
More moderate in complexity, variable cycle time control systems have met with some success. The theory behind such moderate variable cycle time control systems is to sense a few important fractionation variables and to modify the phase cycle times to obtain some of the advantages of a fully variable cycle time operation but with considerably less complexity and cost in the control system.
Among the control theories practiced by such moderate variable cycle time control systems is that of sensing the saturation front as it moves through a particular preselected location in the dessicant bed. Cycle alternation is automatically switched when the saturation front reaches this predetermined location. To sense the saturation front, electrical conductivity and capacitance sensors are frequently located in the dessicant bed at the predetermined location. When the saturation front contacts the sensors, either the electrical conductivity or the capacitance is modified by the interaction with the saturation front. Problems with conductivity and capacitance sensors have occurred because of the effects of corrosion and foreign particles on the sensors over prolonged periods of time, thus reducing the system performance. Dew point sensors have also been used to control cycle times. Dew point sensors have been placed in the exit gas outlet, but dew point sensors give unreliable or faulty readings under certain common conditions of fluctuations in the gas flow rates. Temperature sensors have also been employed. The temperature sensors have been placed in the dessicant beds to sense certain adsorption conditions, since heat is liberated when adsorption occurs.
Some variable cycle control systems also adjust the purge gas flow rate and the regeneration time to fit the degree of adsorption which has occurred immediately previously during the adsorption phase of the dessicant bed. Still other variable cycle control systems create different times for regeneration of one bed compared to the time allowed for adsorption of the other bed at simultaneous points in the cyclic operation of a twin tower fractionation apparatus. All of these various different control arrangements obtain some type of theoretical advantage. However, all variable control systems involve compromises in cost, efficiency, maintenance and convenience of use.